It is often necessary to have the ability to store an exact replica on of an incoming RF signal to be able to replay and utilize it in the future. Unlike tape recorders and other types of recording devices in which audio or text is stored, it is only with great difficulty that one can obtain an incoming RF signal and duplicate it at a time convenient to the user.
In the past, various approaches been utilized to capture incoming signals and generate a replica signal modulated in such a way as to provide a countermeasure, to for instance countermeasure incoming threat missiles. These systems include an RF memory loop based on piezoelectric delay lines, fiber optic delay lines; magnetostrictive delay lines, inductive delay lines composed of strings of inductors and capacitors (Ladder Topology), and traveling wave tube delay lines. There were noted limitations with these approaches starting with the fidelity of the captured signal and eventual loss of signal quality following several passes or recirculation cycles. However, all of these approaches have become impractical and in their place a digital radiofrequency memory (“DRFM”) has been utilized to digitize an incoming RF input signal at a frequency and bandwidth necessary to adequately represent the signal and then to reconstruct that RF signal when required.
A typical DRFM system converts an incoming signal to a digital stream of data which is then phase corrected and stored digitally to be played back when needed. However the DRFM has multiple issues which preclude a simple design. Moreover DRFMs are speed limited and dynamic range limited and are not optimized for size, weight and power.
It will be appreciated that the ability to sample an incoming RF signal and to be able to reproduce it at a later time has a major application in the provision of countermeasure signals which alter the modulation or some other parameter of the incoming signal in such a way as to be able to spoof or countermeasure for instance incoming missiles which can be caused to veer off course. Additionally, the ability to take an original RF signal and to be able to modify it to provide misinformation is indeed useful in various asymmetric warfare situations. Further, the ability to detect an incoming RF signal and to duplicate the signal for analog use thereafter has application in all manner of communications given the ability to faithfully reproduce and use the original RF signal.
Typical digital radio frequency memories occupy a large amount of space in a rack aboard an aircraft which occupy laterally a number of feet, and height wise the same. The amount of power necessary is primarily associated with the powering of analog-to-digital converters, memories and digital-to-analog converters in which the RF signal is not only stored but is phase corrected so as to be able to faithfully reproduce the incoming signal. Were it possible to eliminate the analog-to-digital conversion stage as charge well as the digital-to-analog conversion stage, a large amount of equipment could be dispensed with. This also applies to the elimination of serial to parallel converters and phase correction as charge well as a sampling oscillator.
Thus, a simple storage device that can faithfully store and reproduce an incoming RF signal would be of great advantage. This is especially true in applications where space and power is limited such as for instance in drones. There is therefore a need for an RF memory system that can provide simplicity of design, speed, and dynamic range and can reduce device weight, size and power as compared to the present digital radio frequency memory systems.
Referring now to FIG. 1, a digital radio frequency memory, DRFM, shown at 10 is designed to digitize an incoming RF signal 12 at a frequency and bandwidth necessary to adequately represent the signal and then reconstruct the RF signal at an RF output terminal 14 of the device. The DRFM of FIG. 1 converts the RF signal to a digital stream of data which is phase corrected and stored digitally to be played back when needed.
DRFM 10 includes a limiting amplifier 16 the output of which is coupled to a quadrature IF mixer 18 and thence to low pass filters 20 which output to an analog-to-digital converter 22, the output of which is coupled to a memory subassembly 24. The memory subassembly 24 includes a memory 26 operably coupled to parallel/digital and parallel/serial converters 28 that are clocked by a phase correction circuit 30 driven by a sample oscillator 32. This circuit provides phase correction 34 and clock signals 36 to converters 28. The output of the phase correction circuit 30 is applied to an RF output subassembly 40A that includes a digital-to-analog converter 42A, low pass filters 44, a single sideband modulator 46, amplifier switch 48 and an output port 50 from which is derived an appropriately modulated replica of the incoming RF signal 12. Memory subassembly 24 is controlled by control subassembly 52 that includes a main control 54 and addresses the generation unit 56, a control interface 58 and a voltage regulator 60 which completes a digital radio frequency memory system.
The DRFM is an electronic method for digitally capturing and retransmitting RF signal. DRFMs are typically used in radar jamming, although applications in cellular communications are becoming more common. A DRFM system is designed to digitize an incoming RF input signal at a frequency and bandwidth necessary to adequately represent the signal, and then reconstruct that RF signal when required. The most significant aspect of DRFM is that as a digital “duplicate” of the received signal, it is coherent with the source of the received signal. As opposed to analog ‘memory loops’, there is no signal degradation caused by continuously cycling the energy through a front-end amplifier which allows for greater range errors for reactive jamming and allows for predictive jamming. A DRFM may modify the signal prior to retransmitting which can alter the signature of the false target; adjusting its apparent radar cross section, range, velocity, and angle. DRFMs present a significant obstacle for radar sensors.